1. Field of the Invention
This invention relates to compositions useful for cutoff devices and, more particularly, the thermal cutoff compositions employing a mixture of at least two crystalline organic compounds.
2. Background of the Invention
It is known to provide thermal cutoff assemblies having electrical switching units which change its operating condition when a member disposed therein melts by being heated to a certain temperature. The members typically include what will hereinafter be referred to as a thermal cutoff composition.
Heretofore, thermal cutoff compositions have been formulated from a single organic compound having a known melting temperature commonly in pellet form. These compounds were typically blended with a binder, such as an epoxy, a lubricant such as calcium stearate and/or a pigment such as a metal oxide for purposes of color coding the pellet. For example, U.S. Pat. No. 4,514,718 discloses 4-methylumbelliferone as a preferred organic compound for thermal cutoff applications. While the foregoing thermal cutoff composition has proven useful, significant improvements and advantages are provided by the present invention.
As will be understood by those skilled in the art, the melt temperature is not the only characteristic of thermal cutoff compositions which is important. In addition, the composition should be chemically stable, thermally stable and typically electrically non-conductive in the molten state.
It is a feature of this invention to provide thermal cutoff compositions useful in association with thermal cutoff constructions wherein the composition is capable of being formed into a member adapted to melt (or otherwise changes shape) at a certain temperature or other condition and thereby cause an electrical switching unit of the thermal cutoff construction to change its operating condition.
In particular, it has been found that blends employing at least two organic compounds having known melt temperatures, when brought together via blending or otherwise combined, exhibit improved thermal properties and performance characteristics over the individual organic components. In addition to having the desired melt temperatures, the compositions of the present invention exhibit good chemical and thermal stability. Further, the thermal cutoff compositions are electrically non-conductive in the molten state.
For purposes of the present invention, it is important to note that the phrase xe2x80x9cmelting point temperaturexe2x80x9d as used herein, does not refer to a temperature wherein the product is at the equilibrium point between liquid and solid phases. Rather, melting point temperature is to be considered the temperature at which the thermal cutoff composition (pellet) no longer possesses the structural integrity required to maintain a switch in a held open or held closed position depending on the embodiment. To the extent that such melt temperatures require measurement, various apparatuses such as those produced by Thomas Hoover, Mettler and Fisher-Johns may be employed. Differential Scanning Colorimetry (DSC) techniques are also useful.
Accordingly, it is a primary object of this invention to provide thermal cutoff compositions for forming members employed in thermal cutoff constructions.
It is a further object of the present invention to provide thermal cutoff compositions with defined melt temperature ranges.